Electric control system



y [1939- v H. w. LORD 2,158,878

ELECTRIC CONTROL SYSTEM Original Filed Jan. 2, 1936 2 Sheets-Sheet lu'mmm'om Inventor. Harold W Lord,

byfi His ttovney.

May 16, 1939. w LORD 2,158,878

Original Filed Jan. 2, 1936 2 Sheets-Sheet 2 Inventof HGTOId \N. kovd,byfli yTjuML H His Attorney.

Patented May 16, 1939 UNITED STATES PATENT OFFICE ELECTRIC CONTROLSYSTEM Harold W. Lord, Schenectady, N. Y., assignor to General ElectricCompany, a corporation of New York 13 Claims.

My invention relates to electric control systems and more particularlyto electric valve circuits for controlling dynamo-electric machines.

This application is a division of my copending application Serial No.57,190, filed January 2, 1936, entitled Electric control system, and onwhich United States Letters Patent No. 2,085,596 were granted June 29,1937 and assigned to the assignee of the present application.

Heretofore in electric control systems for electric translatingapparatus, such as dynamo-electric machines and electric valveapparatus, there have been devised numerous arrangements for controllingthe translating apparatus in accordance with an electrical conditionsuch as the voltage of an associated circuit. Where it has been deemedexpedient to control the translating apparatus in ditlerent mannerswithin different ranges of the voltage of an associated circuit, the 20control apparatus required to accomplish this result has been expensiveand complicated in construction and arrangement. For example, in thecontrol of dynamo-electric machines of the synchronous type, there hasbeen evidenced a decided 25 need for control apparatus to maintain anoperating condition, such as the torque of the machine, substantiallyconstant within a predetermined range of voltages of the associatedsupply circuit and for controlling the torque or other 30 operatingcondition in a difi'erent predetemined manner within difl'erent rangesof the voltages of the alternating current supply circuit. Furthermore,it has become evident that control apparatus for performing thisfunction preferably should be of the type which is entirely electricalin operation requiring no moving parts and necessitating only theminimum amount of inspection and a minimum number of replacements.

It isan object of my invention to provide a 40 new and improved'controlsystem for dynamoelectric machines.

Another object of my invention is to provide a new and improved controlsystem for dynamoelectric machines, whereby an operating char- 45acteristic of the machine may be controlled in a predetermined mannerwithin a predetermined range of an electrical condition of an associatedcircuit and whereby the operating condition of the machine may be variedin diflerent predeter- 50 mined manners within difierent ranges of theelectrical condition of the associated circuit.

A further object of my invention is to provide a new and improvedcontrol circuit for dynamoelectric machines of the synchronous typewhere- 55 by the field excitation of a machine is varied or maintainedconstant to eiiect control 01' an operating characteristic of themachine under varying electrical conditions of an associated circuit.

In accordance with the illustrated embodiment of my invention, I providea. control circuit for a dynamo-electric machine of the synchronous typewherein the energization oi the field winding is varied or maintainedconstant to effect control of an operating condition of the machineunder varying electrical conditions, such as the voltage of anassociated alternating current supply circuit. In particular, the fieldwinding of the dynamo-electric machine is energized from the alternatingcurrent circuit through electric valve translating apparatus whichsupplies direct cur-' rent to the field winding. An excitation circuitis employed to control the conductivity of the electric valve means toeifect the desired control of the energization of the field winding. Theexcitation circuit comprises a non-linear resonant circuit of the seriestype which is energized from the alternating current circuit and whichincludes a serially connected resistance, a self-saturable inductanceand a capacitance, and a source of alternating voltage having acomponent corresponding in phase to thevoltage of thealternating currentsupply circuit. The resultant of the voltage appearing across thecapacitance and the alternating voltage provide a resultant periodicpotential which is employed to control the conductivity of the electricvalve means. This resultant periodic potential is impressed on thecontrol members of the electric valve means. By virtue of thenon-linearity of the non-linear resonant circuit, the resultant periodicpotential obtains predetermined difierent phase relationships relativeto the voltage of the alternating current circuit within difierentpredetermined ranges of voltages of the alternating current circuit,thereby providing a means for controlling the electric valve means andthe energization of the field winding in accordance with the voltage ofthe alternating current supply circuit.

For a better understanding of my invention reference may be had to thefollowing description taken in connection with the accompanyingdrawings, and its scope will be pointed out in the appended claims.

In the drawings, Fig. 1 represents diagrammatically an embodiment of myinvention as applied to an electric valve circuit for energizing thefield circuit of a dynamo-electric machine; while Fig. 2 representscertain operating characteristics of apparatus embodying my invention.Diagrams I to V of Fig. 3 also represent certain operatingcharacteristics of the apparatus diagrammatically shown in Fig. 1.

Referring now to Fig. 1 of the drawings, my invention isdiagrammatically shown as applied to an electric valve circuit I forsupplying direct current to a field winding 3 of dynamo-electric machine2, having a three-phase stator winding 4. The electric valve circuit Iand the stator winding 4 are energized from a three-phase alternatingcurrent circuit 5. The electric valve translating circuit I may compriseany suitable valve aggregate for transmiting energy to the field circuit3 and I have shown by way of example a pair of electric valves 6 and I,preferably of the vapor electric type, having anodes 8, cathodes 9 andcontrol electrodes III. Atransformer II, having a secondary winding I2and an electrical mid-tap I2, is energized from the alternating currentcircuit 5 through conductors I3, and is employed to connect the valves 6and I for full wave rectification. The field'winding 3 of machine 2 isconnected to the electric valve circuit I by means of conductors I4.

To provide means for controlling the conductivity of electric valves 6and I, I provide an excitation circuit I5 which responds to anelectrical condition of the alternating current circuit 5, to controlthe voltage impressed upon the respective control electrodes I0 ofvalves 6 and l. The excitation circuit I5 may be arranged to control theelectric valve circuit I in a manner to provide a predeterminedoperating characteristic over a certain range of voltage of the circuit5 and to provide a different operating characteristic over a differentrange of voltages. The excitation circuit I5 comprises a non-linearresonant circuit of the series type employing a capacitance I6, andself-saturating inductance or reactor I1 and an adjustable resistance I8having a movable tap or contact I9. By non-linear resonant circuit, Irefer to that type of circuit which may include a condenser of propercapacitance connected in series with a reactance coil having a closedmagnetic core and a source of alternating electromotive force. Thecurrent in the circuit varies in a non-linear relation to the impressedvoltage. That is, as the voltage in the circuit is gradually increasedthe current increases substantially linearl3 with the voltage until acertain value of voltage is reached at which saturation of the core ofthe inductance takes place and the inductance of the coil resonates withthe capacitance of the condenser. The current then suddenly jumps to avery high value which is several times the value of current prior to thesudden increase. As the electromotive force is increased still further,the current again increases with respect to the impressed voltage in asubstantially linear relation. These circuits may be said to have theproperty of nonlinear, or ferro, resonance. Conductors 20 are employedto connect the non-linear series circuit to one phase of the alternatingcurrent circuit 5. A transformer 2| having a primary winding 22 and asecondary winding 23 and energized from the alternating current circuit5 by conductors 24 is employed to furnish a sinusoidal voltage withwhich an electrical quantity of the nonlinear circuit is combined tocontrol the conductivity of valves 6 and I. The resultant of the voltageappearing across capacitance I6 and the voltage appearing across winding23 of transformer 2| is impressed upon primary winding 26 of transformer25 by means of conductors 28 and 29. The terminals of the secondarywinding 21 are connected to the control electrodes III of electricvalves 6 and I through conductors 30 and self-biasing circuits eachcomprising a current limiting resistance 3| and a capacitance 32. Amid-point 21' of secondary winding 21 is connected to the cathodes 9 ofelectric valves 6 and I by means of a conductor 33. 4

Although my invention is represented as being applied to a system forenergizing the field winding of a dynamo-electric machine, it should beunderstood that it may be applied to electric circuits generally.

The operation of the embodiment of my invention diagrammatically shownin Fig. 1 may be best explained by considering the arrangement whenunidirectional'current is being supplied to the field winding 3 of thedynamo-electric machine 2 from alternating current circuit 5 by means ofelectric valves 6 and 'I and the associated transformer II. If it beassumed that the voltage of circuit 5 is less than the voltage at whichthe non-linear circuit resonates, the arrangement will function as abi-phase rectifier, and there will be substantial phase coincidencebetween the voltages impressed on the control electrodes I0 and thevoltages impressed on valves 5 and I. As will be understood by thoseskilled in the art, if there is substantial phase coincidence betweenthe voltages impressed upon the control electrodes I0 and the voltagesimpressed between the respective anodes 8 and the cathodes 9, theaverage current conducted by the valves will be maximum and the directcurrent voltage will be maximum. As the phase of the voltage impressedupon the control electrodes I0 is retarded relative to the voltageimpressed between the anodes 8 and cathodes 9 of the electric volves 8and I, the average current conducted by the valves will be decreased toeffect a decrease in voltage of the direct current circuit I4,

In the particular arrangement of my invention shown in Fig. 1, theelectric valve circuit I is arranged so that upon decrease in thevoltage of the alternating current circuit 5, the voltage of the directcurrent circuit I4 may be maintained constant or may be increased ordecreased. If it is desired to obtain an increase in the voltage of thedirect current circuit l4 upon decrease in the voltage of alternatingcurrent circuit 5, the excitation circuit I5 is arranged so that duringa normal range of voltageof the alternating current circuit 5 thevoltagesimpressed upon the control electrodes III of electric valves 6and "I lag the voltages impressed between the respective anodes and thecathodes by an appreciable angle or phase displacement. This phaserelationship is accomplished by selecting the constants of thenon-linear circuit so that for operation above a predetermined range ofvoltages of the alternating current circuit 5 the excitation circuit I5effects a retardation in the phase of the voltages impressed on thecontrol electrodes III. The critical resonance voltage of the non-linearcircuit is chosen relative to the Voltage of .circuit 5 so that as thevoltage of circuit 5 decreases, the phase of the potentials impressedupon the control members III is advanced to increase the average currentconducted by the valves, eifecting impressed upon one of the electricvalves, for 75 example, electric valve 6. This voltage is considerablyless than the critical resonance voltage of the non-linear circuit ofexcitation circuit IS. The curve 1) represents the voltage appearingacross the secondary winding 23 of transformer 2I and the curverepresents the Ioltage appearing across the capacitance I6, while thecurve d represents the resultant voltage impressed upon the primarywinding 26 of transformer 25. During operation below the criticalresonance voltage, it will be apparent that the voltage appearing acrossthe capacitance I6,.and represented by curve c, is relatively smallcompared with the other voltages existing in the excitation circuit Iand that the capacitance voltage lags the voltage of circuit 5 bysubstantially 180 electrical degrees. current circuit 5 approaches thecritical resonance voltage of the non-linear circuit, represented by thecurve e of Diagram I, the voltage of capacitance I5 is increased inmagnitude and advanced in phase to effect a retardation of the voltageimpressed upon the control electrode III.

Diagram II represents certain voltages in the excitation circuit as thevoltage of the alternating current circuit 5 approaches the resonancevoltage. Curve f of Diagram II represents the voltage now appearingacross the secondary winding 23 of transformer 2I and curve g representsthe voltage appearing across the capacitance I6, while curve hrepresentsthe resultant voltage impressed upon the primary winding 26 oftransformer 25. It will be understood that as the voltage of thealternating current circuit 5 approaches the critical resonance voltage,the resultant voltage, represented by curve h, impressed upon theprimary winding 25 of transformer 25 is retarded in phase relative tothe voltage of the alternating current circuit 5, and hence effects aretardation in phase of the voltage impressed upon the controlelectrodes III of electric valves 6 and I relative to the anode-cathodepotentials of these values. Under the conditions of operationrepresented by the curves of Diagram I, since there is substantial phasecoincidence between the voltages impressed upon the control electrodesIII of electric valves 5 and 1 and the voltages impressed upon theelectric valves, the output voltage will be maximum for a certainimpressed voltage. However, under the conditions of operationrepresented by the curves of Diagram II, since the voltages impressedupon the control electrodes I0 have been retarded in phase relative tothe voltages impressed between the anodes and cathodes, the averagecurrent conducted by the electric valves will be substan tially lessthan the average current conducted under the conditions of operationrepresented in Diagram I. The voltages impressed upon the direct currentcircuit I4 by the electric valves 5 and I under the operating conditionsrepresented by the curves of Diagram II, that is, when the voltagesimpressed upon the control electrodes I0 lag the voltages impressed uponthe electric valves 6 and I by the transformer II, is represented by thecurve 7 of Diagram III. It should be understood that curve 7' representsthe voltage of circuit I4 when there is appreciable induct- However, asthe voltage of alternatingby a proper choice of constants for thenon-linear circuit comprising capacitance I6, self-saturating inductanceI1 and resistance -I8, it is possible to control the range of phasedisplacement obtainable between" the voltages impressed upon the controlelectrodes I0 of electric valves 5 and I and the voltages impressedbetween the anodes 8 and the cathodes 9 of these valves,and hence toobtain a considerable variation in the control of the voltage of directcurrent circuit I4.

Diagram V represents the voltages appearing in the excitation circuit I5when the voltage of circuit 5 is increased beyond the critical resonancevoltage region where the resistance I8 has a suitable value; curve frepresents the voltage between the voltage 72 impressed upon primary wnding 26 of transformer 25 and the voltage of the circuit 5 as thelatter voltage increases beyond the region of the critical resonancevoltage In Fig. 2 of the drawings there are shown curves representingthe variation in the voltage of the direct current circuit I4 as afunction of the voltage of alternating current circuit 5 for variousvalues of the resistance I 8 where the load circuit I I comprisesinductance. Curves It, 112, n and 0 represent the relation between thevoltage of direct current circuit I 4 and the voltage of alternatingcurrent circuit 5 for different values for the resistance I 8, ofincreasing value in the order named, while curve 32 is merely areference line. By the choice of values for resistance I8 relative tothe voltage of the alternating current supply circuit 5 and the valuesof capacitance I6 and inductance II, it is possible to obtain a varietyof operating characteristics-for the apparatus employed. For example,witha relatively high value of resistance for the element I8,represented by curve 0 of Fig. 2, it is possible to obtain asubstantially constant voltage in circuit I4 for voltages in circuit 5above a predetermined value. From the point q to the point 0 on curve 0,the increase in the direct current voltage is effected by the increasein the voltage impressed upon the electric valves 6 and I. During thisrange of voltages the circuit is operating below the critical resonancevoltage region and as a result thereof there is substantial phasecoincidence between the potentials impressed upon the control electrodesI0 and the potentials impressed between the anodes 8 and the cathodes 9of electric valves 6 and I. At the voltage represented by point 0' ofthe cuit 5 approaches the critical resonance voltage region for theexcitation circuit I5 and effects thereby a retardation in the phase ofthe voltage impressed upon the control electrodes I0. Upon furtherincrease in the voltage of the alternating current circuit 5, there iseffected a further retardation in the voltage impressed upon the controlelectrodes to maintain a substantially constant voltage of the directcurrent circuit I4 represented by the portion of curve 0 beyond point0', By choosing a smaller value of resistance for the element I8, it ispossible to obtain operating characteristics represented by the curvesis, m and n.

The operating characteristic represented by curve 122 of Fig. 2 is ofparticular interest since it shows a substantially linear increase invalue of the direct current voltage for increases in the alternatingcurrent Voltage between the points or and m. Upon further increase inthe alternating current voltage into the region or the criticalresonance voltage of the excitation circuit is, represented by thecurves of Diagram IV of Fig. 3, there is efiected a retardation in thephase displacement of the resultant excitation circuit voltage relativeto the voltage e of the alternating current circuit 5 to eiiect aprogressive decrease in the voltage of direct current circuit l4represented by the portion or the curve between points m and m". It willbe noted that the control of the conductivity of electric valves ii andl in the vicinity of point m" oi curve 112 is continuous and that thetransition from one portion of curve m to another portion thereof iseffected in a smooth and continuous manner. For voltages greater thanthe voltage correspondingto the point m" there is effected a progressiveadvancement in the phase of the voltages impressed upon the controlelectrodes Ill relative to the voltages impressed upon electric valves 6and l, which increases the average current conducted by electric valves6 and l to increase the voltage of the direct current circuit i l. Thephase relation ships of the voltages appearing in excitation circuit 15within the range of voltages represented by m and m of curve m, arerepresented in Diagram IV of Fig. 3, while the phase relationships forvoltages of circuit 5 having a greater value than the represented bypoint m on curve m are shown in Diagram V.

If the constants of the excitation circuit l5 are chosen to obtain anoperating characteristic as represented by the curve n of Fig. 2, theelectric valve circuit i may be made to operate to control theenergization of "the field winding 3 of the dynamo-electric machine 2 tomaintain a predetermined pull-out torque under varying voltageconditions of circuit 5. If the excitation circuit I5 is designed tooperate in the region beyond 12' on. curve n, the excitation circuitwill effect an increase in the voltage impressed upon direct currentcircuit id as the voltage of alternating current circuit decreases. Inthis manner, as the voltage of the alternating current circuit isdecreased, the energization of the field winding will be increased tomaintain a predetermined minimum pull-out torque. Of course, to obtainthis characteristic the voltage of the alternating current circuit 5must be in the region of the critical resonance voltage of thenon-linear circuit of excitation circuit IS. The rates at which I theenergization of the field winding 3 is varied in response to variationsin the voltage of the alternating current circuit 5 may be controlled bychoosing various different values for the resistance it.

While I haveshown and described my invention as applied to a particularsystem of connections and as embodying various devices diagrammaticallyshown, it will be obvious to those skilled in the art that changes andmodifications may be made without departing from my invention, and I,therefore, aim in the appended claims to cover all suchchanges andmodifications as fall within the true spirit and scope of my invention.

aisaevs winding, a translating circuit comprising an electric valve forenergizing said field winding, and means comprising a source ofalternating voltage and a non-linear resonant circuit energized fromsaid alternating current circuit for providing conjointly a pericdicpotential to control the conductivity of said electric valve to changeprogressively the energization of said field winding in one directionfor a predetermined direction of change of an electrical condition ofsaid machine within a predetermined range of said condition and tochange progressively in the opposite direction the energization of saidfield winding for the same direction of change of said condition withina different predetermined range of said condition.

2. In combination, an alternating current circuit, a dynamo-electricmachine having a field winding, electric valve means interposed betweensaid circuit and said field winding for energizing said field winding,and an excitation circuit comprising a source of alternating current anda nonlinear resonant circuit for controlling said electric valve meansfor progressively increasing the energization of said'field winding forvoltages of said alternating current circuit above a predetermined valuefor one direction of change of the voltage of said alternating currentcircuit and for progressively increasing the energization of said fieldwinding for voltages of said alternating current circuit less than saidpredetermined value for the opposite direction of change of the voltageof said alternating current circuit.

3. In combination, an alternating current circuit, a dynamo-electricmachine having a field winding and being energized from said alternatingcurrent circuit, electric valve means for supplying unidirectionalcurrent to said field winding from said alternating current circuit, andmeans comprising a non-linear resonant circuit energized from saidalternating current circuit and a source of alternating voltage forcontrolling the conductivity of said valve means in a predeterminedmanner for a predetermined direction of change of voltage within onerange of voltages of said alternating current circuit and forcontrolling the conductivity of said valve means in a difierent mannerwithin a different range of voltages of said alternating current circuitfor the same direction of voltage change.

4. In combination, an alternating current supply circuit, adynamo-electric machine energized from said alternating current circuitand having a field winding, electric valve means for supplyingunidirectional current to said field winding from said alternatingcurrent circuit, and means comprising a non-linear resonant circuitenergized from said alternating current circuit and a source ofalternating potential for maintaining the, average current conducted bysaid valve means substantially constant within'a predetermined range ofvoltages of said alternating current circuit and for effecting adecrease in the average current conducted by said electric valve meansfor voltages below said predetermined range.

5. In combination, an alternating current circuit, a dynamo-electricmachine connected to be energized from said alternating current circuitand having a field winding, electric valve means interposed between saidalternating current circuit and said field winding for supplying directcurrent to said field winding, and an excitation circuit for controllingsaid electric valve means comprising a non-linear resonant circuitenergized from said alternating current circuit and a source ofalternating voltage for providing conjointly a periodic controlpotential having a progressively increasing phase displacement relativeto a predetermined direction of change of the voltage of saidalternating current circuit within a predetermined range of voltages ofsaid circuit and having a progressively decreasing phase displacementrelative to the same direction of change of voltage of said alternatingcurrent circuit for voltages of said circuit above said predeterminedrange.

6. In combination, an alternating current supply circuit, adynamo-electric machine connected to said alternating current circuitand having a field winding, electric valve means interposed between saidalternating current circuit andsaid field winding for supplying directcurrent to said field winding, and an excitation circuit comprising anon-linear resonant circuit energized from said alternating currentcircuit and a source of alternating voltage of substantial phasecoincidence with the voltage of said alternating current circuit forproviding a periodic control potential for controlling said electricvalve means to decrease progressively the energization of said fieldwinding for a predetermined direction of change of voltage within apredetermined range of voltages of said alternating current circuit andto increase progressively the energization of said field winding for thesame direction of change of voltage within different predeterminedranges of voltages of said alternating current circuit;

7. In combination, an alternating current circuit, a dynamo-electricmachine of the synchronous type having a stator, winding connected tosaid alternating current circuit and having a field winding, electricvalve means including a control member interposed between saidalternating current circuit and said field winding for controlling theenergization of said field winding, and an excitation circuit comprisinga non-linear resonant circuit of the series type energized from saidalternating current circuit and a source of voltage of substantial phasecoincidence with the voltage of said alternating current circuit forproviding a periodic control potential of progressively increasing phasedisplacement relative to the voltage of said alternating current circuitfor controlling said electric valve means to decrease progressively theenergization of said field winding for a predetermined direction ofchange of voltage within a predetermined range of voltages of saidalternating current circuit and of progressively decreasing phasedisplacement relative to-the voltage of said alternating current circuitto increase progressively the energization of said field winding for thesame direction of change of voltage within a difierent predeterminedrange of voltages of said alternating current circuit.

8. In combination, an alternating current circuit, a dynamo-electricmachine of the synchronous type having a stator winding connected tosaid alternating current circuit and having a field winding, electricvalve means interposed between said alternating current circuit and saidfield; winding for supplying direct current to said field winding, andan excitation circuit for controlling the conductivity of said electricvalve means comprising a non-linear resonant circuit energized from saidalternating current circuit and a source of alternating potential ofsubstantial phase coincidence with the voltage of said alternatingcurrent circuit for providing a periodic control potential forcontrolling said electric valve means to maintain the energization ofsaid field winding substantially constant for voltages of saidalternating current circuit above a predetermined range of voltages.

9. In combination, an alternating current circuit, a dynamo-electricmachine having a stator winding energized from said alternating currentcircuit and having a field winding, electric valve means having controlmembers and being interposed between said alternating current circuitand said field winding for supplying direct current to said fieldwinding, and an excitation circuit connected to said alternating currentcircuit for energizing said control members comprising a voltagesensitive circuit for impressing on said control members a periodiccontrol potential having a predetermined phase displacement ofsubstantially constant value relative to the voltage of said alternatingcurrent circuit within a predetermined range of voltages of saidalternating current circuit and for effecting a progressive increase inphase displacement of said periodic control potential for voltages ofsaid alternating current circuit within a different predetermined rangeof voltages and for effecting a progressive decrease in phasedisplacement of said periodic control potentialsfor voltages beyond saidsecond mentioned range of voltages, said displacements occurring for aprogressive increase in the voltage of said alternating current circuit.

10. In combination, an alternating current circuit, a dynamo-electricmachine having a field winding, electric valve means interposed betweensaid circuit and said field winding for energizing said field winding,and an excitation circuit energized from said alternating currentcircuit comprising voltage sensitive means including a source ofalternating voltage and a non-linear resonant circuit for providing aperiodic control voltage to control said electric valve means to effecta progressive increase in the energization of said field winding for apredetermined direction of change of voltage within a predeterminedrange of voltages of said alternating current circuit and to effect aprogressive decrease in the energization of said field winding for thesame direction of voltage change within a predetermined difierent rangeof voltages of said alternating current circuit.

11. In combination, an alternating current circuit, a dynamo-electricmachine having a field winding, electric valve means interposed betweensaid circuit and said field winding for energizing said field winding,and an excitation circuit including a source of alternating voltage anda non-linear resonant circuit energized from said alternating current"circuit and means responsive to a predetermined electrical quantity ofsaid non-linear circuit and the voltage of said source for controllingsaid electric valve means to eflect a progressive increase in theenergization of said field winding within a predetermined range ofvoltages of said alternating current circuit below a predetermined valuefor one direction of change in the voltage of said alternating currentcircuit and to effect a continuous control of the energization of saidfield winding through said voltage of predetermined value and to effecta progressive decrease in the energizationof said field winding within apredetermined different range of voltages of said alternating currentcircuit above said predetermined value for the same direction of changeof the voltage ofsaid alternating current circuit.

12. In combination, an alternating ciurent circuit, a dynamo-electricmachine having a winding connected to said alternatingcurrent circuitand having a field winding forcontrolling said machine, electric valvemeans for controlling the energization of said field winding and havingan anode, a cathode and a control member, and means comprising a sourceof alternating voltage and a non-linear resonant circuit energized inaccordance with an electrical condition of said alternating currentcircuit for impressing on said control member a periodic potentialhaving a progressively increasing phase displacement rela-- tive to thevoltage impressed on said anode to decrease progressively the torque ofsaid machine for a predetermined direction of change of voltage within apredetermined range of voltages of said alternating current circuit andhaving a progressively decreasing phase displacement relative to thevoltage impressed on said anode to increase progressively the torque ofsaid machine ter the same direction of change or" voltage Within apredetermined range of voltages of said alternating current circuitabove said first mentioned range.

13. In combination, an alternating current circuit, a dynamo-electricmachine having a stator L prising a non-linear resonant circuitenergized in accordance with said electrical condition of saidalternating current circuit and a source of voltage of substantial phasecoincidence with the voltage of said alternating current circuit forimpressing on said control member a periodic control potential having asubstantially-constant phase displacement relative to the voltageimpressed on said anode for a predetermined range and direction ofchange of said electrical condition and having a progressivelydecreasing phase displacement relative to the voltage impressed on saidanode to maintain the energization of said field Winding substantiallyconstant within a predetermined range of said electrical condition abovesaid first mentioned-range for the same directionoi change of saidelectrical condition.

HAROLD W. LORD.

